1. Field of the Invention
The present invention relates to exposure techniques, and more particularly, relates to an atmosphere control technique which controls an atmosphere in an airtight chamber of an exposure apparatus by circulating an inert gas having a predetermined purity.
2. Description of the Related Art
Hitherto, in manufacturing of a semiconductor element such as an LSI or a VLSI having an ultra fine pattern, a reduction type projection exposure apparatus has been used in which a circuit pattern drawn in a mask is formed onto a substrate coated with a photosensitive agent by printing a reduction projection technique. Concomitant with the improvement in packaging density of semiconductor elements, development of micropatterning techniques has been increasingly required. Accordingly, along with the advancement of a resist process, exposure apparatuses have also been continuously improved for satisfying the requirement of micropatterning techniques.
As a method for improving the resolving power of an exposure apparatus, there may be mentioned a method for further changing the exposure wavelength to a shorter one and a method for increasing the numerical aperture (NA) of a projection optical system.
As for the exposure wavelength, an i-line having a wavelength of 365 nm was developed, and in recent years, a KrF excimer laser having an oscillation wavelength of approximately 248 nm and an ArF excimer laser having an oscillation wavelength of approximately 193 nm have been developed. Furthermore, development of a fluorine (F2) excimer laser having an oscillation wavelength of approximately 157 nm has also been carried out.
An ArF excimer laser having a UV wavelength, in particular, having a wavelength of approximately 193 nm, and a fluorine (F2) excimer laser having an oscillation wavelength of approximately 157 nm are each known to have an oxygen (O2) absorption band in the vicinity of the wavelength band described above. Hence, in an optical path of an exposure optical system including a reticle or a wafer, in order to suppress the decrease in transmittance of exposure light in the air and the generation of ozone, measures have been taken to reduce the oxygen concentration in the optical path to a level on the order of several ppm or less by a purge device using an inert gas such as nitrogen.
In an exposure apparatus using an ArF excimer laser having a UV wavelength, in particular, a wavelength of approximately 193 nm, or a fluorine (F2) excimer laser having a wavelength of approximately 157 nm, ArF excimer laser beams or fluorine (F2) excimer laser beams are very liable to be absorbed by a substance. Hence, the substance mentioned above present in the optical path must be purged to a level on the order of several ppm or less. The same phenomenon as described above can also be observed when moisture is present, and hence, moisture must also be removed to a level on the order of several ppm or less.
Accordingly, the interior of the exposure apparatus, particularly, the interior of the optical path of ultraviolet rays is purged with an inert gas. In addition, a load-lock mechanism is provided at a portion communicating between the interior and the exterior of the exposure apparatus. When to be carried inside the exposure apparatus, a reticle or a wafer is once shielded from the exterior atmosphere by the load-lock mechanism, and after impurities in the load-lock mechanism are purged away with an inert gas, the reticle or wafer is then carried inside the exposure apparatus.
FIG. 1 is a view showing an example of an exposure apparatus using an inert gas circulating air-conditioning device which is formed based on an air circulating air-conditioning technique adapted to a related exposure apparatus.
In FIG. 1, reference numeral 6 indicates a wafer; reference numeral 7 indicates a movable wafer stage which receives a wafer; reference numeral 8 indicates a structural member supporting the wafer stage 7; reference numeral 9 indicates a projection optical system projecting a pattern drawn in a reticle (not shown) onto the wafer 6; and reference numeral 19 indicates a structural member supporting this projection optical system. In addition, reference numeral 1 indicates an airtight chamber isolating the wafer stage 7 and a part of the projection optical system 9 from the exterior atmosphere, a load-lock 2 is provided beside this airtight chamber, and furthermore, between this load-lock 2 and the wafer stage 7, a wafer transfer system 24 is provided. A wafer carried inside the airtight chamber 1 from the outside through the load-lock 2 is placed on the wafer stage 7 by the wafer transfer system 24, and an exposure process is then to be carried out. In addition, a filter box 20 having a dust collector filter 13 is connected to one end of the airtight chamber 1, the dust collector filter 13 being provided at a nozzle thereof, and the filter box 20 is also connected to an air conditioner 4 through a circulation supply path 21. In addition, this airtight chamber 1 has a return hole R, and this return hole R is connected to the air conditioner 4 through a circulation return path 22. An inert gas in the airtight chamber 1 is sucked through this return hole R and is then returned inside the air conditioner 4 through the circulation return path 22. In the air conditioner 4, a cooling device 10 and a heating device 11 are provided, and for example, after the temperature of an inert gas blown out from the dust collector filter 13 is sensed by a temperature sensor Ts1, the operation of the cooling device 10 and the heating device 11 are controlled by a temperature control device TC so as to always maintain the temperature at a predetermined level. An inert gas passing through the cooling device 10 and the heating device 11 is supplied by a blower 3 to the filter box 20 through the circulation supply path 21. When an inert gas passes through a chemical filter 12 and the dust collector filter 13, organic and inorganic impurity gases are removed by the former and dust is removed by the latter, and the inert gas is again blown into the airtight chamber 1.
Furthermore, an inert gas supply path S3 is connected to the circulation return path 22, and by an inert gas supply device not shown in the figure, an inert gas having a high purity is always supplied in those circulation paths. In addition, an inert gas exhaust path S2 is connected to the circulation supply path 21, an impurity monitoring detector 14a is provided for this exhaust path S2, and a part of the inert gas in those circulation paths is always exhausted outside and is monitored. As described above, since an inert gas having a high purity is always supplied while circulating through the airtight chamber 1 and the air conditioner 4, an inert gas atmosphere can be obtained in the airtight chamber 1 in which predetermined oxygen and moisture concentrations are achieved and in which a predetermined temperature is maintained.
In addition, for example, in Japanese Patent Laid-Open No. 9-218519, in order to decrease the amount of an inert gas, a method has been disclosed which purges a reticle space receiving a reticle stage with an inert gas. According to the method described above, in addition to an inert gas supply source and a supply path, an inert gas circulation path is formed having a blower, a heat exchanger, and a dust collector filter, and after an inert gas in the reticle space and an inert gas supplied from the supply source are mixed together, the mixed inert gas thus obtained is refined by the heat exchanger and the dust collector filter and is then again supplied into the reticle space, so that the amount of the inert gas supplied from the supply source is decreased.
As described above, in an exposure apparatus using UV rays, in particular, using ArF excimer laser beams or fluorine (F2) excimer laser beams, since ArF excimer laser beams or fluorine (F2) excimer laser beams are very liable to be absorbed by oxygen and moisture, in order to obtain satisfactory transmittance and stability, oxygen and moisture must be sufficiently removed.
However, in the case of the related device described above, there has been a problem in that a time required for changing the state in which the airtight chamber 1 and the air conditioner 4 are sufficiently exposed to the atmosphere to the state of a predetermined inert gas atmosphere is very long, that is, a time required for an initial purge is disadvantageously very long. In the initial purge, in general, while not being circulated, an inert gas having a high purity is supplied so that air or a mixture of air and an inert gas present inside is exhausted.
As can be apparently seen in FIG. 1, the case described above is equivalent to that in which an inert gas supplied from the inert gas supply path S3 reaches the inert gas exhaust path S2 while pushing out air present in the apparatus. However, in this case, there are two paths, that is, there are a path communicating with the inert gas exhaust path S2 through the airtight chamber 1 and a path communicating with the inert gas exhaust path S2 through the air conditioner 4. The two paths described above have different volumes from each other, and an inert gas may easily pass through one of the paths as compared to the other path. For example, compared to the volume of the airtight chamber 1 including the filter box 20, the air conditioner 4 has a small volume, and hence an inert gas easily passes therethrough. In this case, since an inert gas supplied from the inert gas supply path S3 preferentially flows through the path at the air conditioner 4 side and is exhausted from the inert gas exhaust path S2, the replacement of air with an inert gas is readily carried out at the air conditioner 4 side. On the other hand, air at the airtight chamber 1 side spreads toward the air conditioner 4 side primarily by a diffusion phenomenon, in other words, an inert gas at the air conditioner 4 side spreads toward the airtight chamber 1 side primarily by a diffusion phenomenon, and the air is gradually replaced with an inert gas. As described above, of the two paths in which the replacement is performed with an inert gas, the replacement is dominantly carried out by a diffusion phenomenon in one of the two paths, and as a result, a very long time is required for the initial purge on the whole. Accordingly, the time required for startup of an apparatus is increased, and as a result, the productivity thereof is disadvantageously degraded.
In addition, a large amount of moisture is adsorbed onto surfaces of substances exposed to the atmosphere. In particular, since many resins have hydrophilic properties or moisture absorption properties, moisture is unlikely to be desorbed from the surface of a resin as compared to the surface of a metal, and hence, a problem may arise in that a large amount of moisture adsorbed on a resin is continuously desorbed for a long period of time. Even if a fluorinated resin having a relatively superior moisture desorption property is used, it was confirmed by the inventor of the present invention that the moisture desorption property of a fluorinated resin is inferior to that of a metal by one order of magnitude or more.
However, many types of resin materials have been used for various units forming an exposure apparatus. In particular, resins must be used as pipe materials and coating materials for cables in view of assembly density and workability. Hence, in a reticle space or a wafer space in which resin members are inevitably provided, even when the purge is performed with an inert gas having a high purity, moisture adsorbed onto the surface of the materials is gradually being desorbed for a long period of time, and as a result, it will take several tens of hours to enable the moisture to reach a predetermined concentration. Accordingly, the time required for startup of an apparatus is increased, and as a result, the productivity thereof is disadvantageously degraded. Furthermore, in order to obtain a predetermined moisture concentration, a larger amount of an inert gas having a high purity must be continuously supplied from the supply source, and as a result, the running cost of the apparatus is disadvantageously increased.
In addition, when the inert gas circulation path is formed in a reticle space or a wafer space, the dust collector filter must be provided upstream of the aforementioned space. This dust collector filter has a structure in which a fabric filter medium intrinsically having a large surface area is repeatedly folded and placed in a housing so that a large amount of a gas flow is processed with a low pressure loss, and the total surface area of the fabric filter medium with which the gas flow is brought into contact is tremendously large. Furthermore, since a glass or a fluorinated resin is used as the filter medium, a large amount of moisture adsorbed onto the surface thereof is disadvantageously being desorbed for a long period of time. In addition, whenever necessary, the chemical filter may be provided upstream of this dust collector filter in some cases. As a filter media of the chemical filter, for example, an ion exchange resin, a granular or fabric active carbon, or an inorganic ceramic may be used; however, as is the case of the dust collector filter described above, the surface thereof is large, and as a result, a large amount of moisture adsorbed onto the surface of the filter medium is disadvantageously being desorbed for a long period of time. For example, active carbon adsorbs moisture at a content of approximately 5% to 50% on a weight basis at 25° C. and at a relative humidity of 40% to 80%. In a chemical filter containing, for example, 1 kg of active carbon in a dry state, approximately up to 500 cc of absorbed moisture is present, and when this filter is placed in a dry atmosphere at a relative humidity of approximately 3%, most of the moisture thus adsorbed is evaporated in the atmosphere. Of course, onto other chemical filters and dust collector filters, a considerable amount of moisture is adsorbed, and the same phenomenon as described above surely occurs. After the dust collector filter or the chemical filter described above is used, although the purge is carried out with an inert gas having a high purity in a reticle space or a wafer space, water molecules adsorbed onto the surface of the filter medium are being desorbed for a long period of time, and it will take several tens of hours to enable the moisture to reach a predetermined concentration. Accordingly, the time required for startup of the apparatus is increased, and as a result, the productivity thereof is disadvantageously degraded.
Furthermore, in order to obtain a predetermined moisture concentration, a larger amount of an inert gas having a high purity must be continuously supplied from the supply source, and as a result, the running cost of the apparatus is disadvantageously increased. In addition, besides moisture, impurities in the atmosphere and in a clean room may be adsorbed onto filters and the like during the transportation and the assembly in some cases, and as a result, the impurities themselves or those dissolved in moisture adsorbed on the surfaces of the filters and the like disadvantageously behave in the same manner as that of the moisture described above.
In addition, when maintenance or repair of various units placed in a reticle space or a wafer space is performed, the space must be exposed to the atmosphere. In this case, at the same time, the dust collector filter and the chemical filter are also exposed to the atmosphere. As a result, exterior moisture and impurities are again adsorbed onto the surfaces of the filter media of the filters, and even when the purge is re-started using an inert gas after the maintenance is completed, a long period of time and a large amount of an inert gas having a high purity are required to obtain a predetermined moisture concentration and impurity concentration, and as a result, the degradation in productivity of the apparatus and rapid increase in running cost thereof disadvantageously occur.
Furthermore, when an exposure apparatus is operated while an inert gas is circulated inside the airtight chamber, wafers and reticles are frequently carried into the airtight chamber. Of course, in this case, outside air is prevented from entering the airtight chamber by the use of a load-lock. However, on the wafer surface, the surface of a resist provided thereon, and the surface of a reticle, moisture and impurities in the exterior atmosphere are being adsorbed, and before the moisture and impurities thus adsorbed are desorbed to obtain an ideal equilibrium state, the wafer and the reticles are generally carried into the airtight chamber. Otherwise, the productivity required for an exposure apparatus cannot be ensured. Hence, since moisture and impurities are always carried inside the airtight chamber during operation by the wafer and the reticle carried into the exposure apparatus, most of the moisture and impurities are adsorbed onto the surfaces of the filter media of the chemical filter and the dust collector filter while they pass through the circulation paths, and as a result, as the amount of adsorption is increased, the concentration of moisture and impurities in the airtight chamber are gradually increased. This increase in concentration of the moisture and impurities causes the degradation in transmittance in an exposure light atmosphere and the generation of haze of optical members, and as a result, a problem may arise in that the productivity of the exposure apparatus is degraded.
Accordingly, development has been desired in which, while an inert gas is circulated, an inert gas concentration at a predetermined level is obtained in a wafer space or a reticle space within a short period of time required for startup by the use of a small amount of an inert gas having a high purity, and in which the predetermined concentration level described above is then maintained.
Furthermore, development has also been desired in which the concentrations of moisture and impurities in an inert gas circulating atmosphere, which are increased during the operation, are recovered to a predetermined level within a short period of time.